1. Field of the Invention
The present invention relates to a waveguide having an attenuation pole waveguide bandpass filter, and more particularly relates to a waveguide having an attenuation pole waveguide bandpass filter to improve skirt characteristics of passband.
2. Description of the Related Art
Conventionally, attenuation pole waveguide bandpass filters with various shapes and structures have been proposed.
For example, a prior art in Unexamined Japanese Patent Heisei 07-058505 (Laid-open) discloses, as shown in FIG. 22, an attenuation pole waveguide bandpass filter which arranges two or more cylindrical posts 5 along the longitudinal direction of the radio wave propagation direction such that it can determine a center frequency and band width of a passband by varying the intervals between these cylindrical posts 5, the width of the cylindrical posts 5 or the width and height of the waveguide.
Likewise, Unexamined Japanese Patent 2004-289352 (Laid-open) also discloses a waveguide having an input and output structure with attenuation poles. As shown in FIG. 23, this waveguide arranges resonators 12A, 12B and 12C constituting a three component filter inside a generally rectangular dielectric block. Grooves (Irises) are formed between those resonators 12A, 12B and 12C, so that the frequency and bandwidth of the passband can be determined.
In addition, “IEEE Antennas and Propagation Society International Symposium and USNC/CNC/URSI North American Radio Science Meeting Columbus, Ohio” Jun. 22-27, 2003 also discloses an waveguide having attenuation pole waveguide bandpass filters. As shown in FIG. 24, this waveguide has two or more attenuation pole waveguide bandpass filters arranged along the radio wave propagation direction, such that it can determine the passband by changing the type of these attenuation pole waveguide bandpass filters.